Whenever light travels from one medium into another with a different index of refraction, reflection occurs. The amount of reflection depends on the index difference, or contrast. In applications where light travels from one high index medium, such as glass, into air, which has a low index, and into another high index medium again, significant reflections can occur. Both anti-reflection films or index matching techniques have been used to suppress reflections arising from index contrast.
In fiberoptic communications systems, optical subassemblies are used as light sources, receivers or processing devices. A receiver optical subassembly is exemplified by an optical waveguide, such as an optical fiber and a photodetector. Often, as necessitated by environmental and construction considerations, the optical waveguide must be placed far away from the photodetector. Since laser light that leave the optical waveguide will diverge, one or more lenses are placed between the waveguide and photodetector to collect the divergent light beam and focus it onto the photodetector. This optical path introduces multiple, high optical index contrast interfaces. At every boundary between different media, such as fiber/air, air/lens, lens/air, air/photodetector, reflection occurs due to the high index change. These reflections are lost energies that degrade the signal strength. Present techniques used to reduce the amount of signal loss include antireflection coating of individual components and the addition of a thick film of index matching media on the photodetector.
Furthermore, in some applications, such as analog signal transmission, multiple reflections can cause signal noise. Extra care is needed to prevent reflection from the receiver optical subassembly back into the optical fiber waveguide. For example, return loss in analog CATV transmission system requires that the receiver return loss to exceed 40 dB. Typically, the fiber is angle-cleaved at 6-8 degrees to achieve the required return loss.
Accordingly, a new apparatus and method are needed to reduce signal strength degradation.